1. Field of the Invention
The present invention relates to digital wireless communications systems of the type which employ portable, battery-powered communications devices, such as remote telemeters worn by ambulatory hospital patients for monitoring purposes. More particularly, the present invention relates to a network architecture, and an associated TDMA (time division multiple access) communications protocol, for facilitating the efficient and reliable exchange of information between portable wireless devices and centralized monitoring stations.
2. Description of the Related Art
Medical telemetry systems that allow the physiologic data of multiple, remotely-located patients to be monitored from a central location are known in the art. These systems typically comprise remote telemeters that remotely collect the physiologic data of respective patients and transmit the data over a wireless link to a centralized monitoring station. This physiologic data may include, for example, real-time electrocardiograph (ECG) waveforms, CO2 levels, and temperature readings. From the centralized monitoring station, a clinician can visually monitor the physiologic status, in real time, of many different patients. The central station may also run automated monitoring software for alerting the clinician whenever a predetermined physiologic event occurs, such as a cardiac arrythmia condition.
Remote telemeters of medical telemetry systems are generally of two types: instrument remote telemeters and ambulatory remote telemeters. An ambulatory remote telemeter is a portable, battery-powered device which permits the patient to be monitored while the patient is ambulatory. The ambulatory telemeter attaches to the patient by a strap or other attachment device, and receives the patient""s physiologic data via ECG leads (and/or other types of sensor leads) which attach to the patient""s body. The physiologic data is continuously transmitted to the central monitoring station by the telemeter""s RF (radio frequency) transmitter to permit real-time monitoring. (A design of a remote transceiver which may be used in a two-way, ambulatory telemeter is described in the above-referenced provisional application.) Instrument remote telemeters operate in a similar manner, but receive the patient""s physiologic data from a bedside monitor (or other instrument) over a hardwired link, such as an RS-232 connection. Instrument remote telemeters that transfer the physiologic data to the central station over a hardwired connection are also common.
One problem that is commonly encountered in the field of medical telemetry involves signal loss caused by multi-path interference. Multi-path interference is a well-known phenomenon which occurs when a signal takes two or more paths (as the result of signal reflections) from the transmitter to the receiver such that the multi-path components destructively interfere with each other at the receiver""s antenna. To reduce the effects of multi-path interference, some telemetry equipment manufactures have included multiple antenna/receiver pairs on each remote telemeter. With this technique, known as spacial diversity, when one of the antennas experiences multi-path fading, the other antenna (and the corresponding receiver) is used to receive the signal. One problem with this method is that it adds to the cost, size and complexity of the remote telemeter. In addition, in at least some implementations, a loss of data may occur when a xe2x80x9cswitch-overxe2x80x9d is performed from one antenna/receiver pair to the other.
Another problem that has been encountered in the field of medical telemetry relates to the ability to monitor a large number of patients over a coverage area that extends to all patient areas of the hospital. A common solution to this problem involves installing a large number of antennas (e.g., 200 or more) throughout the hospital (with different antennas positioned in different patient areas), and interconnecting the antennas using signal combiners to form a single, distributed antenna system. One problem with this xe2x80x9cdistributed antenna systemxe2x80x9d approach is that each antenna and its associated preamplifier (or preamplifiers) contributes to the noise floor of the antenna system, and thereby increases the minimum transmit power at which the transmitting components of the system can operate. (The reasons for this noise floor degradation are discussed below.) Consequently, unless the transmission power of the system""s transmitters is increased, a practical limitation is imposed on the number of antennas that can be included in the system, and on the coverage area provided by the system.
Although the noise floor degradation problem can potentially be overcome by increasing the transmission power of the telemetry equipment, there are at least two problems associated with increasing the transmit power. The first problem is that under existing Federal Communications Commission (FCC) regulations, medical telemetry equipment is only permitted to operate within certain frequency bands, and must operate within certain prescribed power limits within these bands. Under FCC Part 15.241, for example, which governs the protected VHF (174-216 MHz) medical telemetry band (a band which is generally restricted to VHF television and medical telemetry), telemetry devices are not permitted to transmit at a signal level which exceeds 1500 microvolts/meter at 3 meters. To operate at power levels which exceed this maximum, frequency bands which offer less protection against interference must be used. The second problem is that increasing the transmit power of an ambulatory telemeter will normally produce a corresponding reduction in the telemeter""s battery life.
Another problem with distributed antenna systems is that they are typically highly vulnerable to isolated sources of electromagnetic interference (xe2x80x9cEMIxe2x80x9d). Specifically, because the signals received by all of the antennas are combined using RF signal combiners, a single source of interference (such as a cellular phone or a faulty preamplifier) at or near one of the antennas can introduce an intolerable level of noise into the system, potentially preventing the monitoring of all patients. One consequence of this problem is that antennas generally cannot be positioned near known intermittent sources of EMI such as X-ray machines, CAT (computerized axial tomography) scanners, and fluoroscopy machines, preventing patient monitoring in corresponding diagnostic areas.
In light of these and other problems with existing medical telemetry systems, the present invention seeks to achieve a number of performance-related objectives. One such objective is to provide an architecture in which the coverage area and patient capacity can be increased without degrading the noise floor. This would allow the telemetry system to be expanded in size and capacity without the need to increase the transmit power of the battery-powered remote telemeters, and without the need to operate outside the protected VHF medical telemetry band. A related objective is to provide an architecture which is highly scalable, so that the capacity and coverage area of the system can easily be expanded through time.
Another goal of the invention is to provide extensive protection against signal drop-outs caused by multi-path interference. The present invention seeks to achieve this objective without the need for multiple antennas or receivers on the telemeters, and without the loss or interruption of physiologic data commonly caused by antenna/receiver switch-overs. A related goal is to provide a high degree of protection against isolated sources of EMI, and to allow patients to be remotely monitored while near known intermittent sources of interference.
Another goal of the invention is to provide an architecture in which a large number of patients (e.g., 500 to 800 or more) can be monitored using a relatively narrow range of RF frequencies (such as the equivalent of one or two VHF television channels). This would allow the RF communications components of the system to be optimized for narrow-band operation, which would in-turn provide a performance advantage over wide-band systems.
In accordance with these and other objectives, a medical telemetry system is provided which includes multiple remote telemeters (which may include both ambulatory and instrument telemeters) which transmit the real-time physiologic data of respective patients via RF to multiple ceiling-mounted transceivers, referred to as xe2x80x9cVCELLs.xe2x80x9d The VCELLs are hardwire-connected to a real-time data distribution network which includes at least one centralized monitoring station. (In a preferred implementation, each group of 16 VCELLs is connected via twisted pair lines to a respective xe2x80x9cconcentrator PC,xe2x80x9d and the concentrator PCs and monitoring stations are interconnected as part of a hospital local area network.)
The VCELLs are distributed throughout the hospital such that different VCELLs provide coverage for different patient areas, and are spaced such that the coverage zones provided by adjacent VCELLs overlap with one another. Different VCELLs within the same general area communicate with the remote telemeters on different respective RF frequencies (i.e., frequency channels), so that a remote telemeter can selectively communicate with a given VCELL by selecting that VCELL""s frequency. As described below, however, VCELL frequencies are reused by VCELLs that are spaced sufficiently apart from one another to avoid interference, allowing the system to be implemented as a narrow-band system which uses a relatively small number of frequencies (e.g., 10) to provide coverage for an entire hospital facility.
In a preferred embodiment, the remote telemeters communicate with the VCELLs using a wireless time division multiple access (TDMA) protocol in which each VCELL can concurrently receive the real-time physiologic data of up to six remote telemeters (corresponding to six patients). As part of this protocol, the remote telemeters implement a VCELL xe2x80x9cswitch-overxe2x80x9d protocol in which the telemeters establish wireless connections with different VCELLs based on periodic assessments (made by the telemeters) of the wireless links offered by the different VCELLs. Thus, as a patient moves throughout the hospital, the patient""s remote telemeter may connect to (and disconnect from) many different VCELLs.
In operation, the remote telemeters send data packets (during assigned timeslots) to the respective VCELLs with which the telemeters have established wireless connections. (As described below, each remote telemeter preferably remains connected to two different VCELLs at-a-time to provide extensive protection against multi-path interference.) These data packets include the real-time physiologic data of respective patients, and include ID codes which identify the remote telemeters. The VCELLs in-turn forward the data packets to the real-time data distribution network to permit the real-time monitoring of the patients of the system.
To provide protection against multi-path interference and other causes of data loss, each remote telemeter maintains wireless connections with two different VCELLs at-a-time, and transmits each data packet to both of the VCELLs. These duplicate packet transmissions to the two different VCELLs take place on different frequencies during different TDMA timeslots. The two VCELLs forward the data packets to a centralized node (which may be a monitoring station or a concentrator PC in the preferred embodiment), which performs error correction by selecting between the corresponding packets based on error detection codes contained within the packets. Thus, the patient""s physiologic data is sent from the remote telemeter to the centralized node over two separate data paths. Because the two VCELLs are spaced apart, and because the duplicate packets are transferred to the VCELLs on separate frequencies at different times, the packet transfers benefit from the protection offered by spacial diversity, frequency diversity and time diversity.
The architecture of the above-described medical telemetry system provides numerous advantages over prior art systems. One such advantage is that the system can be expanded in patient capacity and coverage area, by the addition of VCELLs, without increasing the noise floor of the system beyond the natural thermal noise floor. (This is because the data signals received by the VCELLs are multiplexed digitally at baseband, rather than being combined by RF analog signal combiners.) Thus, unlike distributed antenna telemetry systems, the noise floor does not impose an upper limit on the size of the system. Moreover, the architecture can accommodate a large number of patients (e.g., 500 to 800 or more) using a low maximum transmission power, such as the maximum transmit power permitted by the FCC for operation within the VHF medical telemetry band.
Another advantage is that the architecture is highly immune to isolated sources of EMI. A source of EMI (such as a cellular phone), for example, will typically contaminate the signals received by no more than one or two nearby VCELLs, as opposed to introducing noise into the entire system. (Because the remote telemeters connect to two VCELLs at-a-time, and automatically switch to different VCELLs when bad link conditions are detected, the contamination of one or two VCELLs will typically result in little or no loss of telemetry data.) One benefit of this immunity is that VCELLs can be installed within X-ray rooms and other radiological diagnostic rooms which contain intermittent sources of EMI, allowing patients to be monitored in such areas.
Another advantage of the architecture is that it permits the reuse of RF frequencies by VCELLs that are sufficiently spaced apart (by about 500 feet in a VHF implementation) to avoid interference with each other. By extending this concept, the present invention provides coverage for the entire facility using a relatively small number of frequencies which fall within a relatively narrow frequency band. In a preferred VHF implementation, for example, it is estimated that a typical hospital can be covered using only 10 to 12 VCELL frequencies which fall within a frequency band that is equal in width to about two adjacent VHF television channels. This characteristic of the architecture advantageously allows the telemeter transceivers to be optimized (through the appropriate selection of transceiver components) for a relatively narrow band of frequencies, which in-turn improves performance.
The present invention thus comprises a medical telemetry system for monitoring patients within a medical facility. The system comprises a plurality of radio frequency (RF) transceiver units connected to a wired computer network to provide wireless access points to the wired computer network. The RF transceiver units are spatially distributed throughout patient areas of the medical facility to provide multiple coverage zones for monitoring patients. The system further includes a plurality of ambulatory telemeters that connect to respective patients, sense at least the ECG signals of such patients, and transmit the ECG signals by wireless communications to the wired computer network via the plurality of RF transceiver units to allow the ECG signals to be monitored substantially in real time on the wired computer network. The telemeters and the RF transceiver units implement a communications protocol in which an RF transceiver unit used to convey a patient""s ECG signals to the wired computer network is selected dynamically, preferably based on assessments of wireless link conditions between the patient""s ambulatory telemeter and specific RF transceiver units, whereby general connectivity is maintained between the telemeter and the wired computer network as the patient moves throughout the multiple coverage zones. In a preferred embodiment, the telemeters and the RF transceiver units communicate using a TDMA protocol which provides a combination of space, frequency and time diversity on wireless transmission of ECG data.
The present invention also provides a method of conveying real-time ECG waveform data of a patient from a telemeter attached to the patient to a wired computer network. The method comprises, with the telemeter: (a) continuously receiving and packetizing the ECG waveform data of the patient; (b) monitoring transmissions from RF transceiver units on each of multiple wireless communications channels, and based on assessments of the transmissions, selecting an RF transceiver unit with which to establish a wireless connection, wherein the RF transceiver units are spatially distributed throughout at least a portion of the medical facility, and are configured to forward ECG waveform data received from telemeters to the wired computer network; (c) establishing a wireless connection with the RF transceiver unit selected in (b); (d) repeating (b) and (c) to maintain general connectivity to the wired network as the patient moves into and out of range of specific RF transceiver units; and (e) transmitting packets of the ECG waveform data to at least one RF transceiver unit with which a wireless connection is currently established to permit the ECG waveform data to be monitored continuously and in real time on the wired computer network.
The present invention further comprises an RF transceiver adapted to be mounted to a ceiling within a patient area of a medical facility to provide wireless access to a wired computer network on which physiologic data of patients is monitored substantially in real time. The RF transceiver comprises a processor programmed to implement a TDMA protocol in which the RF transceiver maintains wireless connections with multiple patient telemeters at a time, and forwards packetized physiologic data received from the multiple telemeters to a processing node of the wired computer network substantially in real time. The processor is further programmed to establish and discontinue wireless connections with the patient telemeters to support patient mobility within the medical facility into and out of range of the RE transceiver.